A semiconductor laser diode basically comprises a body of a semiconductor material or materials having a waveguide region and a clad region on each side of the waveguide region. Within the waveguide region is a region, such as a quantum well region, in which photons are generated when the diode is properly biased by and electrical current. The clad regions are doped to be of opposite conductivity type and are of a material having a lower refractive index than the material of the waveguide region so as to attempt to confine the photons to the waveguide region.
In the design of laser diodes heretofore made and known to those skilled in the art as being of optimum design, the thickness of the waveguide region was limited in extent, usually to be in the order of 0.2 to 0.3 micrometers (μm), so as to achieve a minimization of the threshold current. To achieve the minimization of the threshold current, a substantial overlapping of the optical mode generated in the waveguide region into the adjacent doped regions, such as the dad regions, occurred. Although a major portion of the optical mode generated in the waveguide region remains and travels along the waveguide region, a portion of the optical mode at each end thereof extends into, i.e., overlaps into, the regions of the diode adjacent the waveguide region. This typically results in undesirable optical propagation losses. The propagation loss in a clad region contributes to the propagation loss of the lasing mode to the extent of the propagation loss of said clad region multiplied by the overlap factor of the clad region by the lasing mode. The overlap factor of a clad region is the proportion of photons which are carried in the clad region. Throughout this specification the term “propagation loss” means the propagation loss of the lasing mode. Thus, the overall efficiency of the device is reduced, thereby limiting directly and indirectly the output power capability of the device. Another constant on typical semiconductor laser diodes heretofore made has been the length of the diode, i.e., the distance between its ends. The longer the laser diode, the lower the thermal and electrical resistance of the diode and therefore, in general, the larger the output power. However, because of the lower efficiency resulting from the propagation losses, the length of the laser diode has been limited.
High efficiency, high power lasers have long been pursued for such applications as optical pumping of solid state and fiber laser, direct material processing, printing, communications, sensing, etc. Therefore, it would be desirable to improve the efficiency and reduce the losses of such laser diodes so as to increase the output power of the devices.